Is it [OH]^2 = 1.2e-11/.001

Yes.

The other approach is when we try to calculate concentrations in saturated solution.

Say, we have PbCl

_{2} dissolving:

PbCl

_{2} Pb

^{2+} + 2Cl

^{-}and we know

Ksp=[Pb

^{2+}][Cl

^{-}]

How do we find how much PbCl

_{2} dissolved? Let's say solubility is x mol/L. That means saturated solution contains x mol/L of Pb

^{2+}, but - because of the stoichiometry - twice as much Cl

^{-}. So

[Pb

^{2+}] = x

[Cl

^{-}] = 2x

Now plug these into the Ksp and you will see where these strange coefficients come from.